It is known that the eye receives light which is ideally focused onto the retina through the cumulative convergence provided by the cornea, the lens, and fluids on the eyeball. Refractive errors affect the focus point of the light such that for nearsighted (myopic) people, the focus falls short of the retina allowing images close to the eye to be clearly viewed while blurring images at a distance. Conversely, farsightedness (hyperopic) also fails to provide a focus. Refractometry allows correction via lenses to affect changes due to hyperopia, myopia, and astigmatism, among others.
In certain populations, detection of refractive errors can be performed to prevent irreversible loss of vision. In preschool children, a malady known as amblyopia, otherwise known as "lazy eye", is particularly important. Approximately 5 percent of all children are either born with or develop a form of this malady. As an untreated child having this malady becomes older, neural development in the brain tends to permanently suppress vision in the diseased eye. This prevents proper binocular vision in the child, with this impairment becoming irreversible after about seven years of age. Current medical academy guidelines suggest diagnosis and detection should be made as early as possible, and suggest physicians target the age of three as the proper screening age. A critical aspect to this screening is measurement of the refractive state of the patient's eyes for comparison to known standards.
Photorefractors are known devices which use the red reflex produced by the eye as a method for determining refractive error. These devices produce a flash, either on or off axis with a peephole, and record the resulting reflex which is reflected from the retina. For example, U.S. Pat. No. 4,989,968, issued to Freedman, discloses a small slit aperture and a light source positioned 0.5 mm from the slit aperture. In use, a photograph is taken of the red reflex of a single subject which appears at the aperture when the light source is flashed. A second photograph is taken with the slit aperture and light source rotated 90 degrees from a position used to take the first photograph. Examination of the shape and intensity of the red reflexes which appear in these photographs allows a trained user to detect whether a large refractive error is present.
The primary problem with this device in serving the screening needs of small children is that the interpretation of the result (a photograph) is both time consuming and subjective, making diagnosis impractical on a real time basis. The interpretation consists of identifying the presence of a "defect", but does not provide quantitative information concerning the refractive state, therefore making it difficult to diagnose children or others whose condition is marginal. The device suffers from a "dead zone" which is a refractive range, close to zero diopters, in which no reading is produced. This prevents measurement of a percentage of the target population. In addition, the device also falls short of being compact and held in a single hand during use, as is desirable in the field for screening applications. Another problem with the instant device is that it does not perform effectively on patients having smaller pupils. This prevents a segment of children from being measured in a normal screening environment.
Variations of these devices include CCD camera-based systems. For example, in U.S. Pat. No. 5,632,282, issued to Hay, there is a described method of automatically analyzing the intensity levels of the retinal reflection from a subject's eyes to determine whether pathologic conditions are present. If implemented by a data processor, this method addresses the real time problems confronted by the device of Freedman. However, the problems of "dead zone", lack of quantitative results, compactness of the size of the instrument employing this method, and the requirement for large pupil size are not addressed.
There are known autorefractors which can determine, quantitatively, to a limited extent, refractive differences between the ideal and an aberrative eye.
In known devices, such as manufactured by Nikon, the device requires a number of moving optical elements in order to null the patient's refractive error to obtain a reading. This nulling increases the refractive range which can be measured at the expense of test time and accuracy within the range. These commercial autorefractors require very short working distances to achieve measurement.
Therefore, a problem which exists concerns the adaptability of such apparatus. Typically, young children are distressed when presented with diagnostic instruments close to their face. This makes devices, such as the "Retinomax" manufactured by Nikon, for example, unusable on many children. In children which will tolerate such a device, the close proximity of the instrument causes accommodation, which tends to make the readings less accurate. The refractive state of an eye depends on what the patient is focusing on. For example, when a patient focuses on a near object, his or her refractive state would appear to be near sighted (myopic), even though the patient may not be. The refractive state which is of interest for screening, as well as for prescribing corrective lenses is the refractive state when the patients eye is relaxed and he or she is focused at a distance. Ideally, this distance is infinity, but in practice, it varies from 20 feet to 6 feet for acuity tests, and approximately 65 cm in retinoscopy evaluations.
Adult patients can be instructed to look at a distant object, such as a chart on a wall, even though objects or people may be closer to them. However, the natural instinct of children is to focus on the closest object which in the screening environment is typically the practitioner or the device. Consequently, the closer the device is to a child patient, the more he or she will accommodate, and the more erroneously myopic the refractive measurement will be compared to the true value. The moving optical elements make the time required for examination longer, which is also less suitable for the low attention span of young children. It is also desirable to achieve higher refractive assessment accuracy in the specific ranges for target populations. For example, American children typically have refractive errors in the range of -1 Diopters to +4 Diopters at age three. Adults are more typically myopic and average approximately -3 Diopters (+1 Diopters to -5 Diopters typically).